Synthetic voacangine

ABSTRACT

Synthetic voacangine, including in substantially enantiomerically enriched forms, and derivatives thereof are provided.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application and claims priority to U.S. patent application Ser. No. 13/749,594 filed on Jan. 24, 2013, which claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application Ser. Nos. 61/590,741 filed Jan. 25, 2012, and 61/591,200 filed Jan. 26, 2012, each of which is hereby incorporated by reference into this application in its entirety.

FIELD OF THE INVENTION

This invention relates to processes for preparing synthetic voacangine, and salts thereof, intermediates thereto, and to compositions comprising the same.

STATE OF THE ART

Voacangine is an alkaloid found in plants such as Tabernanthe Iboga and Voacanga Africana, and has the following structure:

It is an iboga alkaloid which can serve as a precursor for the semi-synthesis of ibogaine:

which can be demethylated to provide noribogaine:

Noribogaine and its pharmaceutically acceptable salts have recently received significant attention as a non-addictive alkaloid useful in treating drug dependency (U.S. Pat. No. 6,348,456) and as a potent analgesic (U.S. Pat. No. 7,220,737). Voacangine is a potential source for making noribogaine. However, plant derived voacangine is problematic because of its limited and unpredictable supply. Furthermore, plant derived voacangine may contain unwanted alkaloids which may find their way to the noribogaine produced from the plant derived voacangine.

Accordingly, there is an ongoing need to provide synthetic voacangine, which can be intermediates in the synthesis noribogaine, preferably in an enantiomerically enriched form.

SUMMARY OF THE INVENTION

This invention provides synthetic voacangine. In one embodiment, this invention provides (+) voacangine, (+) ibogaine, or (+) noribogaine, in a substantially enantiomerically enriched form. In some embodiments, the voacangine, ibogaine, or noribogaine provided herein contains less than 1 ppt, preferably less than 0.9 ppt C¹⁴, more preferably, less than 0.75 ppt, and still more preferably, less than 0.2 ppt. In some embodiments, the voacangine is present as a racemic or scalemic mixture. As used herein, a “scalemic mixture” is a mixture of enantiomers at a ratio other than 1:1. In some embodiments, the voacangine is present in a substantially enantiomerically enriched form.

Also provided here are methods for producing synthetic voacangine and derivatives thereof, including in racemic, or in substantially enantiomerically enriched form. In particular, such derivatives include substitution at the 12 and/or 18 position of voacangine. When substitution is solely at the 12 position, such substitution is other than a methoxy group In some embodiment, the synthetic voacangine are obtained in a substantially enantiomerically enriched form.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A, 1B, and 1C illustrate ¹H-NMR spectra in CDCl₃ of synthetic (+) voacangine, synthetic (+) ibogaine, and synthetic (+) noribogaine prepared according to this invention.

FIGS. 2A, 2B, and 2C illustrate ¹H-NMR spectra in CDCl₃ of synthetic (−) voacangine, synthetic (−) ibogaine, and synthetic (−) noribogaine prepared according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to synthetic voacangine, and substantially enantiomerically enriched forms thereof. However, prior to describing this invention in greater detail, the following terms will first be defined.

It is to be understood that this invention is not limited to particular embodiments described. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of this invention will be limited only by the appended claims.

It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a solvent” includes a plurality of such solvents.

Definitions

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. As used herein the following terms have the following meanings.

As used herein, “(−)” enantiomer refers to the levorotatory enantiomer, and “(+)” enantiomer refers to the dextrorotatory enantiomer.

As used herein, “alkenyl” refers to hydrocarbyl groups having from 2 to 10 carbon atoms and at least one and up to 3 carbon carbon double bonds. Examples of alkenyl include vinyl, allyl, dimethyl allyl, and the like.

As used herein, the term “alkyl” refers to hydrocarbon groups having from 1 to 20, 1 to 6, or 1 to 3 carbon atoms. The alkyl group may contain linear or branched carbon chains. This term is exemplified by groups such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, t-butyl, n-pentyl and the like.

As used herein, the term “alkoxy” refers to —O-alkyl.

As used herein, “alkynyl” refers to hydrocarbyl groups having from 2 to 10 carbon atoms and at least one and up to 2 carbon carbon triple bonds. Examples of alkynyl include ethynyl, propargyl, dimethylpropargyl, and the like.

As used herein, “amino” refers to —NR^(x)R^(y) wherein each R^(x) and R^(y) independently is hydrogen, C₁-C₆ alkyl, C₂-C₆ alkenyl, C₂-C₆ alkynyl, or R^(x) and R^(y) together with the nitrogen atom they are bonded to form a 5-10 membered heterocyclyl ring containing 1-2 nitrogen and/or oxygen atoms, which heterocyclyl ring is optionally substituted with 1-3, preferably, 1-2, or more preferably, a single, C₁-C₃ alkyl group.

As used herein, the term “aryl” refers to an aromatic hydrocarbon ring having preferably 6 ring carbon atoms such as phenyl. “Substituted aryl” refers to aryl substituted with 1-3 C alkoxy or C ₁-C₆ alkyl groups.

As used herein, the term “comprising” or “comprises” is intended to mean that the compositions and methods include the recited elements, but not excluding others. “Consisting essentially of” when used to define compositions and methods, shall mean excluding other elements of any essential significance to the combination for the stated purpose. Thus, a composition consisting essentially of the elements as defined herein would not exclude other materials or steps that do not materially affect the basic and novel characteristic(s) of the claimed invention. “Consisting of” shall mean excluding more than trace elements of other ingredients and substantial method steps. Embodiments defined by each of these transition terms are within the scope of this invention.

As used herein, “C_(x)” refers to a group having x carbon atoms, wherein x is an integer, for example, C₄ alkyl refers to an alkyl group having 4 carbon atoms.

As used herein, “ee” refers to enantiomeric excess and is expressed as (e¹−e²)% where e¹ and e² are the two enantiomers. For example, if the % of e¹ is 95 and the % of e² is 5, then the e¹ enantiomer is present in an ee of 90%. The ee of an enantiomer in a mixture of enantiomers is determined following various methods well known to the skilled artisan, such as using chiral lanthanide based nuclear magnetic resonance shift reagents, forming derivatives with chiral compounds such as chiral hydroxyacids, amino acids, and the like. Various physical measurements such as circular dichroism, optical rotation, etc. are also useful is determining the ee of a mixture of enantiomers.

As used herein, “heterocyclyl” or heterocycle refers to a cycloalkyl group of from 1 to 10 carbon atoms and 1 to 4 heteroatoms selected from the group consisting of oxygen, nitrogen, sulfur within the ring, wherein the nitrogen and/or sulfur atom(s) of the heteroaryl are optionally oxidized (e.g., N-oxide, —S(O)— or —S(O)₂—), provided that the ring has at least 3 and up to 14, or preferably from 5-10 ring atoms. Such heterocyclyl groups can have a single ring or multiple condensed rings wherein the condensed rings may not contain a heteroatom and/or may contain an aryl or a heteroaryl moiety, provided that the point of attachment is through an atom of the non-aromatic heterocyclyl group. Examples of heterocyclyl include pyrrolidinyl, piperadinyl, piperazinyl, and the like. Heterocyclyl rings are preferably saturated, though, heterocyclyl rings including 1-2 carbon carbon double bonds are also contemplated provided that the ring is not aromatic.

As used herein, the term “protecting group” or “Pg” refers to well known functional groups which, when bound to a functional group, render the resulting protected functional group inert to the reaction to be conducted on other portions of the compound and the corresponding reaction condition, and which, at the appropriate time, can be reacted to regenerate the original functionality under deprotection conditions. The identity of the protecting group is not critical and is selected to be compatible with the remainder of the molecule. In one embodiment, the protecting group is an “amino protecting group” which protects the amino functionality of voacangine intermediates during the voacangine synthesis described herein. Examples of amino protecting groups include, for instance, benzyl, acetyl, oxyacetyl, carbonyloxybenzyl (Cbz), and the like. In another embodiment, the protecting group is a “hydroxy protecting group” which protects the hydroxyl functionality of voacangine intermediates during the voacangine synthesis described herein. Examples of hydroxyl protecting groups include, for instance, benzyl, p-methoxybenzyl, p-nitrobenzyl, allyl, trityl, dialkylsilylethers, such as dimethylsilyl ether, and trialkylsilyl ethers such as trimethylsilyl ether, triethylsilyl ether, and t-butyldimethylsilyl ether; esters such as benzoyl, acetyl, phenylacetyl, formyl, mono-, di-, and trihaloacetyl such as chloroacetyl, dichloroacetyl, trichloroacetyl, trifluoroacetyl; and carbonates such as methyl, ethyl, 2,2,2-trichloroethyl, allyl, and benzyl. Additional examples of amino and hydroxy protecting groups may be found in standard reference works such as Greene and Wuts, Protective Groups in Organic Synthesis., 2d Ed., 1991, John Wiley & Sons, and McOmie Protective Groups in Organic Chemistry, 1975, Plenum Press. Methods for protecting and deprotecting the phenolic hydroxyl group of the compounds disclosed herein can be found in the art, and specifically in Greene and Wuts, supra, and the references cited therein.

As used herein, the term “reaction conditions” refers to details under which a chemical reaction proceeds. Examples of reaction conditions include, but are not limited to, one or more of following: reaction temperature, solvent, pH, pressure, reaction time, mole ratio of reactants, the presence of a base or acid, or catalyst, etc.

As used herein, the term “salt” refers to salts derived from a variety of organic and inorganic counter ions well known in the art and include, by way of example only, when the molecule contains an acidic functionality, counter ions such as lithium, sodium, potassium, calcium, magnesium, ammonium, tetraalkyl ammonium, and the like, and when the molecule contains a basic functionality, counter ions such as acetate, citrate, chloride, bromide, lactate, mesylate, maleate, oxalate, phosphate, succinate, sulfonate such as methane sulfonate or para toluenedulfonate, tartrate and the like.

As used herein, “substantially enantiomerically enriched” refers to an enantiomer in an enantiomeric mixture with at least 90% ee, preferably 95% ee, or more preferably 98% ee.

As used herein, the term “voacangine” refers to the compound of formula:

and salts thereof wherein R^(A) is C₁-C₆ alkyl optionally substituted with 1-3 aryl groups, or R^(A) is H, and includes all stereoisomers at the 2, 4, 6, and 18 position, and salts of each thereof. Of particular interest are compounds wherein R^(A) is C₁-C₄ alkyl, and of more particular interest is the compound wherein R^(A) is methyl.

Compounds and Compositions

This invention provides synthetic voacangine compositions which are enantiomerically enriched.

In one aspect, this invention provides synthetic voacangine and voacangine derivative compounds of formula:

or salts thereof wherein,

R¹ is —COOR², —CH₂—OH, or —CH₂—OC(═O)R³;

R² is a metal cation, a C₁-C₆ alkyl optionally substituted with 1-3 phenyls or substituted phenyls, or with a hydroxy, —NHCOCH₃, or an amino group, where the substituted phenyl is substituted with 1-3, C₁-C₆ alkyl and/or C₁-C₆ alkoxy group, or R² is hydrogen;

R³ is an asymmetric hydrocarbyl group such that R³COOH is a chiral carboxylic acid;

R⁴ is C₁-C₆ alkyl, C₁-C₆ alkyl substituted with 1-3, halo, C₁-C₆ alkoxy, phenyl, or substituted phenyl, where the substituted phenyl is substituted with 1-3, C₁-C₆ alkyl and/or C₁-C₆ alkoxy group, or R⁴ is another hydroxyl protecting group.

In a particular embodiment, R¹ is a carboxylate salt that is —COO⁻Li⁺ which is a stable, recoverable salt of the corresponding carboxylic acid.

In another embodiment, R² is C₁-C₄ alkyl. In a preferred embodiment, R² is methyl. In another embodiment, R² is C₁-C₆ alkyl substituted with 1-3 phenyls or substituted phenyls, where the substituted phenyl is substituted with 1-3, C₁-C₆ alkyl and/or C₁-C₆ alkoxy group.

In another embodiment, R¹ is —CH₂—OH. In another embodiment, R¹ is —CH₂—OOCR³.

In certain embodiments, the synthetic compounds provided by this invention are provided in substantially enantiomerically enriched or diastereomerically enriched form.

Compounds wherein R¹ is —COOR² can be converted to noribogaine via treatment with aqueous hydrazine, and compounds wherein R¹ is —CH₂—OH can be converted to noribogaine under retro aldol condensation conditions, which reaction conditions will be apparent to the skilled artisan in view of this disclosure.

Certain preferred compounds of this invention are of formula:

wherein R¹ is CO₂(CH₂)₂OH, CO₂(CH₂)₂NHCOCH₃, CO₂(CH₂)₂NMe₂,

The synthetic voacangine, its substantially enantiomerically enriched forms, and other synthetic compounds of this invention are distinguished from such plant derived compounds (e.g., and without limitation, voacangine isolated from plant sources) by its ¹⁴C content. ¹⁴C has a half-life of about 5,730 years and is generated in the upper atmosphere as ¹⁴CO₂. The amount of ¹⁴CO₂ present is approximately 1 ppt (parts per trillion) and, through photosynthesis, accumulates in plants resulting in a ¹⁴C content of plant material of approximately 1 ppt. Accordingly, plant derived voacangine is expected to have approximately 1 ppt ¹⁴C. Conversely, the synthetic compounds disclosed herein are derived from fossil fuels, which, due to ¹⁴C decay, would have a ¹⁴C content of less than 1 ppt ¹⁴C. Accordingly, provided herein is synthetic voacangine, ibogaine or noribogaine, preferably in the (+) form, or a voacangine derivative having a ¹⁴C content of less than 1 ppt, preferably, less than 0.95 ppt, or more preferably less than 0.8 ppt. In one embodiment, provided herein is synthetic voacangine or a voacangine derivative having a ¹⁴C content of less than 0.6 ppt, or less than 0.5 ppt, or less than 0.4 ppt, or less than 0.3 ppt, or less than 0.2 ppt, or less than 0.1 ppt. In another embodiment, provided herein is synthetic voacangine or a voacangine derivative having a ¹⁴C content of 0.8 ppt to 0.95 ppt or 0.7 ppt to 0.95 ppt. The amount of ¹⁴C can be analyzed using methods well known in the art (i.e. radiocarbon analyses can be carried out according to the American Society for Testing Materials ASTM D6866 procedure (ASTM international, 100 Barr Harbon Drive, PO Box C700, West Conshohocken, Pa. 19428-2959)). Furthermore, provided is a method for distinguishing synthetic voacangine or a voacangine derivative from plant derived voacangine or voacangine derivatives based on their respective ¹⁴C content.

Synthetic Methods

The compounds of this invention can be prepared from readily available starting materials using the following general methods and procedures. It will be appreciated that where typical or preferred process conditions (i.e., reaction temperatures, times, mole ratios of reactants, solvents, pressures, etc.) are given, other process conditions can also be used unless otherwise stated. Optimum reaction conditions may vary with the particular reactants or solvent used, but such conditions can be determined by one skilled in the art by routine optimization procedures.

Additionally, as will be apparent to those skilled in the art, conventional protecting groups may be necessary to prevent certain functional groups from undergoing undesired reactions. Suitable protecting groups for various functional groups as well as suitable conditions for protecting and deprotecting particular functional groups are well known in the art. For example, numerous protecting groups are described in T. W. Greene and G. M. Wuts, Protecting Groups in Organic Synthesis, Third Edition, Wiley, N.Y., 1999, and references cited therein.

Furthermore, the compounds of this invention will typically contain one or more chiral centers. Accordingly, if desired, such compounds can be prepared or isolated as pure stereoisomers, i.e., as individual enantiomers or diastereomers, or as stereoisomer-enriched mixtures. All such stereoisomers (and enriched mixtures) are included within the scope of this invention, unless otherwise indicated. Pure stereoisomers (or enriched mixtures) may be prepared using, for example, optically active starting materials or stereoselective reagents well-known in the art. Alternatively, racemic mixtures of such compounds can be separated using, for example, chiral column chromatography, chiral resolving agents and the like.

The starting materials for the following reactions are generally known compounds or can be prepared by known procedures or obvious modifications thereof. For example, many of the starting materials are available from commercial suppliers such as Aldrich Chemical Co. (Milwaukee, Wis., USA), Bachem (Torrance, Calif., USA), Emka-Chemce or Sigma (St. Louis, Mo., USA). Others may be prepared by procedures, or obvious modifications thereof, described in standard reference texts such as Fieser and Fieser's Reagents for Organic Synthesis, Volumes 1 15 (John Wiley and Sons, 1991), Rodd's Chemistry of Carbon Compounds, Volumes 1 5 and Supplementals (Elsevier Science Publishers, 1989), Organic Reactions, Volumes 1 40 (John Wiley and Sons, 1991), March's Advanced Organic Chemistry, (John Wiley and Sons, 4th Edition), and Larock's Comprehensive Organic Transformations (VCH Publishers Inc., 1989).

Synthetic voacangine and 12-substituted derivatives thereof can be prepared as shown in the non limiting illustration below. For illustrative purposes only, the following discussion will illustrate reactions where R⁴ is methyl.

where R⁴ is C₁-C₆ alkyl, C ₁-C₆ alkyl substituted with 1-3, halo, C₁-C₆ alkoxy, phenyl, or substituted phenyl, where the substituted phenyl is substituted with 1-3, C₁-C₆ alkyl and/or C₁-C₆ alkoxy group, or R⁴ is another hydroxyl protecting group.

Compounds wherein R¹ is COOR² other than COOMe are synthesized by employing the corresponding COOR² group or by converting a compound such as Compound 17 to the corresponding COOR² ester, such as by refluxing with the corresponding R²OH, as will be apparent to the skilled artisan.

According to the schemes above, compound 1 (200 g) was converted to compound 3 (120 g) by contacting compound 1 with compound 2 in an inert solvent such as methanol at an elevated temperature such as 80° C. The reaction product was recovered by conventional methods to provide for compound 3 in 43% yield. Compound 3 (130 g, 90% pure) was converted to compound 4 (60 g, 90% pure) by contacting compound 3 with pyridine at an elevated temperature, e.g., at a temperature where the pyridine refluxes, preferably for about 0.5 h. Without further separation, compound 4 (60 g), was converted to compound 5 (30 g, 90% pure) by contacting compound 4 with NABH₃CN in acetic acid (AcOH) at room temperature, preferably for about 2 hours. Without further separation, compound 5 (30 g) obtained in the previous step was converted to compound 6 (15 g, 90% pure) by contacting compound 5 with benzaldehyde and NABH₃CN in an inert solvent such as methanol at room temperature. Compound 6 was separated by column chromatography to yield compound 6 (15 g, 90% pure). Compound 7 was converted to compound 11 following a number of steps well known to the skilled artisan and described e.g., in Kuehne et al., J. Org. Chem., 50:919 (1985), incorporated herein in its entirety by reference. In the schemes above, the benzyl (Bn) protecting group can be replaced with other amino protecting groups well known to the skilled artisan.

As shown above, compound 6 was converted to compound 13 (30 g, 90% pure after column chromatographic separation) by contacting compound 6 with of compound 11, preferably under an inert N₂ atmosphere. Without further purification, Compound 13 (30 g) was contacted with of NaBH₄ and mL of AcOH at elevated temperature, such as 90° C., preferably for about 2 hours to yield compound 14 (20 g) after column chromatographic separation. Compound 14 (12 g) was deprotected by contacting with palladium/carbon and hydrogen in AcOH to yield compound 15 (10 g, 80% pure) in 90% purity. Compound 15 was contacted in darkness with of 10% HCl in an inert solvent such as acetonitrile, preferably for about 3 h to provide compound 16 (5 g) in 70% purity.

Compound 16 (0.5 g) was converted, without further purification, under cyclization conditions to racemic synthetic voacangine, compound 17 (2 g, 98% pure). Under the cyclization conditions, compound 16 was refluxed in darkness, in an inert solvent such as benzene for about 10 h. Throughout the synthesis, purity of the product was determined by high performance liquid chromatography optionally with mass spectrometry. The ¹³C nuclear magnetic resonance (NMR) spectra and ¹H NMR spectra of the synthetic compound 17, where R4 is methyl (see, FIG. 1) and a standard sample demonstrated that synthetic compound 17 was indeed voacangine. For each of the steps, the contacting is continued until the reaction substantially complete, as determined by a variety of methods well known to the skilled artisan, such as thin layer chromatography and ¹H-NMR. Certain preferred contacting times are provided herein. Compound 17 was separated by chiral supercritical fluid chromatography into 1 g each of the enantiomers, the naturally occurring (−) voacangine and the non-natural (+) voacangine enantiomer, in substantially enantiomerically enriched form.

In one aspect, this invention provides a process for preparing a compound of formula:

and/or its enantiomer, or a salt of each thereof wherein

R² is C₁-C₆ alkyl optionally substituted with 1-3 aryl groups and R⁴ is C₁-C₆ alkyl, C₁-C₆ alkyl substituted with 1-3, halo, C₁-C₆ alkoxy, phenyl, or substituted phenyl, where the substituted phenyl is substituted with 1-3, C₁-C₆ alkyl or C₁-C₆ alkoxy group, or R⁴ is a hydroxyl protecting group, comprising subjecting a compound of formula:

or a salt thereof to cyclization conditions to provide the compound of formula:

and its enantiomer, or a salt of each thereof.

The compound of formula:

is refluxed in an inert solvent, preferably in absence of light. Suitable solvents include, without limitation benzene and the like. The refluxing is carried out for a period of time sufficient to form a substantial amount of the product.

In one embodiment, the process further comprises contacting the compound of formula:

and/or its enantiomer (i.e., a racemic or scalemic mixture of the enantiomers), or a salt of each thereof, wherein R² and R⁴ are defined as in the process above, under reduction conditions to provide a compound of formula:

and/or its racemate, or a salt thereof. Suitable reduction conditions are well known the skilled artisan and include, contacting with a borohydride or and aluminum hydride, in an inert solvent such as ether or tetrahydrofuran, followed by aqueous work-up.

The compound of formula:

along with its enantiomer, are esterified with a chiral carboxylic acid R³COOH to provide an ester compound of formula:

and its diastereomer. The diastereomeric forms of the ester compound can be separated, and the separated diastereomeric forms hydrolyzed to provide enantiomerically enriched:

As will be apparent to the skilled artisan, the methods of making synthetic voacangine and other synthetic compounds as provided herein also require routine steps of separation and purification, which are performed by column chromatography, crystallization, and the like, as also well known to the skilled artisan. Enantiomerically enriched synthetic voacangine, or an enantiomerically enriched intermediate thereto or an enantiomerically enriched derivative thereof as utilized and provided herein is contemplated to be obtained, inter alia, by chiral chromatographic separation, and/or resolution via diastereomeric salt formation, and/or separation of diastereomeric derivatives. Chiral acids and bases suitable for resolving synthetic voacangine or an intermediate or derivative thereto will be well known to the skilled artisan.

EXAMPLES

These examples illustrate the conversion of resolved (−) and (+) voacangine to the corresponding (−) and (+) noribogaine.

Resolved (+) voacangine (200 mg, 1 equivalent) and 1-dodecanethiol (1.5 equivalent) in dimethyl formamide (DMF, 1.2 mL) was added to a mixture of sodium tertiary butoxide (1.5 equivalent) in DMF (0.8 mL) at 100° C. and the reaction mixture stirred in the dark at 110-120° C. for 6 h and then at room temperature for about 12 h. Volatiles were removed in vacuum, and after aqueous work-up, (+) ibogaine was isolated by extraction with dichoromethane. The organic portion was washed with water and dried over MgSO₄. Volatiles were removed to provide ibogaine as a foamy solid (140 mg). A solution of ibogaine thus obtained, in dichloromethane (DCM, 1.4 mL), was added to a 1 molar BBr₃ (1.5 equivalent) solution in DCM at 0-5° C. over a 2 h period to provide a suspension, which was stirred at room temperature for 12 h. Then, the reaction mixture was cooled to 0-5° C. and MeOH (0.6 mL) was added to it drop wise over a period of 15 minutes and the resulting mixture stirred at room temperature for 12 h. Volatiles were removed in vacuum, and the residue was separated by column chromatography on silica gel using 5% MeOH/CHCl₃ as the eluent to obtain (+) noribogaine (70 mg) as a foamy solid. Specific rotations determined for the naturally occurring (−) enantiomers, and the synthetic (+) enantiomers made according to this invention are tabulated below, which demonstrate the stereochemistry and enantiomeric purity of the synthetic enantiomers prepared according to this invention.

Specific rotation Enantiomer Natural Synthetic Voacangine −42°, c = 1, in +41.3°, c = 1, in chloroform chloroform Ibogaine −48.5°, c = 1, in +47.9°, c = 1; in water water Noribogaine −36.4°, c = 1, in +36.2°, c = 1, in water water

UTILITY

(−) Voacangine has utility in preparing (−) noribogaine, which is useful for treating drug dependency and as an analgesic. See U.S. Pat. Nos. 6,348,456 7,220,737, supra. The voacangine derivatives provided here are also useful for preparing noribogaine. (+) Voacangine and (+) ibogaine is useful for preparing and (+) noribogaine. It is contemplated that (+) noribogaine has utility for treating pain and addiction in a manner similar to (−) noribogaine. It is further contemplated that derivatives of voacangine are useful for testing the the role of opioid receptors in overcoming pain. 

1-5. (canceled)
 6. A compound of formula:

or a salt thereof wherein, R¹ is —COOR², —CH₂—OH, or —CH₂—OC(═O)R³; R² is a metal cation, a C₁-C₆ alkyl optionally substituted with 1-3 phenyls or substituted phenyls, or with a hydroxy, —NHCOCH₃, or an amino group, where the substituted phenyl is substituted with 1-3, C₁-C₆ alkyl and/or C₁-C₆ alkoxy group, or R² is hydrogen; R³ is an asymmetric hydrocarbyl group; R⁴ is C₁-C₆ alkyl provided that when R² is methyl, benzyl, or a metal cation, R⁴ excludes methyl; C₁-C₆ alkyl substituted with 1-3, halo, C₁-C₆ alkoxy, phenyl, or substituted phenyl, where the substituted phenyl is substituted with 1-3, C₁-C₆ alkyl and/or C₁-C₆ alkoxy group; or R⁴ is another hydroxyl protecting group.
 7. A compound of claim 6 of formula:

wherein R¹ is CO₂(CH₂)₂OH, CO₂(CH₂)₂NHCOCH₃, CO₂(CH₂)₂NMe₂, 